1. Field of the Invention
This invention pertains generally to systems for determining location and, more particularly, to systems for determining location and location uncertainty of railroad vehicles.
2. Background Information
In the art of railway signaling, traffic flow through signaled territory is typically directed by various signal aspects appearing on wayside indicators or cab signal units located on-board railway vehicles. The vehicle operators recognize each such aspect as indicating a particular operating condition allowed at that time. Typical practice is for the aspects to indicate prevailing speed conditions.
For operation of this signaling scheme, the track is typically divided into cascaded sections known as “blocks.” These blocks, which may be generally as long as about two to about five miles in length, are electrically isolated from adjacent blocks by typically utilizing interposing insulated joints. When a block is unoccupied, track circuit apparatus connected at each end are able to transmit signals back and forth through the rails within the block. Such signals may be coded to contain control data enhancing the signaling operation. Track circuits operating in this manner are referred to as “coded track circuits.” One such coded track circuit is illustrated in U.S. Pat. No. 4,619,425. When a block is occupied by a railway vehicle, shunt paths are created across the rails by the vehicle wheel and axle sets. While this interrupts the flow of information between respective ends of the block, the presence of the vehicle can be positively detected.
In the case of trains, control commands change the aspects of signal lights, which indicate how trains should move forward (e.g., continue at speed; reduce speed; stop), and the positions of switches (i.e., normal or reverse), which determine the specific tracks the trains will run on. In dark (unsignaled) territory, forward movement of trains is specified in terms of mileposts (e.g., a train is given the authority to move from its current location to a particular milepost along its planned route), landmarks or geographic locations. Sending the control commands to the field is done by an automated traffic control system, or simply control system. Control systems are employed by railroads to control the movements of trains on their individual properties or track infrastructures. Variously known as Computer-Aided Dispatching (CAD) systems, Operations Control Systems (OCS), Network Management Centers (NMC) and Central Traffic Control (CTC) systems, such systems automate the process of controlling the movements of trains traveling across a track infrastructure, whether it involves traditional fixed block control or moving block control assisted by a positive train control system.
In dark territory, controlling the movements of trains is effected through voice communication between a human operator monitoring the control system and the locomotive engineer. The interface between the control system and the field devices can either be through control lines that communicate with electronic controllers at the wayside that in turn connect directly to the field devices, or, in dark territory, through voice communication with a human, who manually performs the state-changing actions (e.g., usually switch throws).
It is known to employ a Global Positioning System (GPS) to determine the position of a train. For example, U.S. Pat. No. 4,899,285 discloses a system in which measurement results of a GPS position measuring apparatus are evaluated to determine whether they are reliable with respect to those derived by an integration calculation position measuring apparatus. The integration apparatus includes a direction sensor using a gyroscope or geomagnetic sensor and a vehicle speed sensor. Three GPS positions are sequentially measured, which correspond to three positions measured by the integration apparatus. The integration apparatus determines whether the measurement results of the GPS apparatus are twice continuously highly reliable. If so, then the integration apparatus adopts the subsequently measured GPS result as the reference position and executes the subsequent measurement of the position of the vehicle.
U.S. Pat. No. 5,129,605 discloses a wheel tachometer that generates pulses for a dead reckoning filter of a train control computer (TCC) to determine speed. The TCC compares velocity and position data, and rejects inconsistent data. A GPS receiver also generates a speed and position signal, which is input to the TCC to indicate position and speed, and also to calibrate the wheel tachometer. The TCC determines the best source of the speed signals. In making such determinations, the GPS speed is generally preferred when it is greater than ten miles per hour or when wheel slip is detected; otherwise, GPS calibrated wheel tachometer speed is used.
U.S. Patent Application Publication No. 2005/0065726 discloses that inertial sensors are subject to low frequency bias and random walk errors. Such errors grow in an unbounded manner upon integrating accelerometer and gyro output signals to obtain velocity and position, i.e., the computation has poor long-term stability. These long-term errors are corrected for by blending with D/GPS data, which possess comparatively excellent long-term stability. Conversely, a conventional navigator solution possesses good short-term stability, as the integration process tends to smooth high-frequency sensor errors (which are usually attenuated significantly by low-pass filtering), while D/GPS data has comparatively poor short-term stability due to, for example, multi-path effects and broadband noise. A train location system and method of determining track occupancy utilizes inertial measurement inputs, including orthogonal acceleration inputs and turn rate information, in combination with wheel-mounted tachometer information and GPS/DGPS position fixes to provide processed outputs indicative of track occupancy, position, direction of travel and velocity. Various navigation solutions are combined together to provide the desired information outputs using a Kalman filter or similar Bayesian estimator.
U.S. Pat. No. 5,902,351 discloses a vehicle tracking system including an inertial measurement unit having at least one gyro and at least one accelerometer, an odometer/tachometer, a GPS receiver, a tag receiver, and a map matching system. A Kalman filter may be utilized to reduce error within the vehicle tracking system and improve the accuracy thereof.
U.S. Pat. No. 5,893,043 discloses a process and an arrangement for determining the position of a vehicle moving on a given track by using a map matching process. At least three types of position measuring data in the form of object site data, path length data and route course data are obtained. A computer unit carries out, for each type of measuring data, a data correlation with a stored desired data quantity for the determination of position results, which are evaluated in an “m-out-of-n” decision making process. In this process, a given number “m” of the “n” determined position results is taken into account.
There is room for improvement in systems for determining location and location uncertainty of railroad vehicles.